Level shift circuits (also referred to herein as level shifters) change the voltage level of a signal. For example, when an output voltage of a first circuit differs from an operating range of a second circuit, a level shifter is arranged between the first circuit and the second circuit, and the voltage level of the signal between the first and second circuits is adjusted. For example, in the driving circuitry for a liquid crystal display (LCD) panel, such as those using thin film transistors (TFT), signals having various voltages may be employed. In this case, signals having different voltages are generated from a common power supply and a level shift circuit is provided to adjust the levels of the signals between circuit blocks having different voltages.
FIG. 1 illustrates a conventional level shifter 100. Such a level shifter 100 may be common in integrated circuit (IC) processes in which the supply voltages do not exceed transistor maximum gate-to-source voltages (Vgs). Level shifter 100 includes PMOS transistor 110 operably coupled (e.g., connected) in series with NMOS transistor 120 such that the drains of the two transistors are operably coupled. The source of NMOS transistor 120 is operably coupled to ground. The source of the PMOS transistor 110 is operably coupled to the supply voltage (Vhigh). The supply voltage Vhigh may also be referred to herein as the shift voltage, as it is the voltage that the level shifter 100 uses to adjust (i.e., shift) the voltage level on the output signal (Vout).
Level shifter 100 also includes NMOS transistor 130 operably coupled in series with PMOS transistor 140, such that the drains of the two transistors are operably coupled. The source of the PMOS transistor 140 is operably coupled to the supply voltage Vhigh. The source of NMOS transistor 130 is operably coupled to gnd (ground). PMOS transistor 110 and PMOS transistor 140 are cross coupled with the gates of each transistor operably coupled to the drain of the other transistor. An input signal (Vin) is applied to level shifter 100 at the gate of NMOS transistor 130. Input signal Vin is also applied to an inverter 105 operably coupled to the gate of NMOS transistor 120.
In operation, when Vin is asserted, NMOS transistor 120 is turned off. NMOS transistor 130 is turned on, which pulls the gate of PMOS transistor 140 to ground, turning on PMOS transistor 110. This causes the output signal (Vout) to be equal to Vhigh. When Vout is equal to Vhigh, PMOS transistor 140 is held off, allowing PMOS transistor 110 to remain on. With Vin asserted, the voltage level of an input signal Vin is shifted to result in an output signal equal Vout to Vhigh.
On the other hand, when Vin is not asserted, NMOS transistor 120 is turned on, which causes Vout to be pulled to ground. NMOS transistor 130 is not turned on which, in addition to the PMOS transistor 140 being held on, holds PMOS transistor 110 off by holding the supply voltage Vhigh at the gate of PMOS transistor 110.
Level-shifting with conventional level shifter 100 may require little, if any, standby current. However, shoot-through current IST may be possible during transitions of Vout, because low-side NMOS transistors 120 and 130 must overcome the current of high-side PMOS transistors 110 and 140, respectively, in order to switch (i.e., change or toggle) the output signal Vout. Shoot-through current IST may arise when PMOS transistor 110 and NMOS transistor 120 are momentarily on due to asymmetries in the rise and fall times of the PMOS transistor 110 and the NMOS transistor 120. This mode of switching may also inhibit the transition time thereby increasing propagation delay and output slew rate.
The Vgs of the transistors is an important aspect of level-shifting circuits such as level shifter 100. These transistors may be required to standoff the entire supply voltage (i.e., shift voltage Vhigh). For level shifters with relatively low-voltages for a supply voltage, such a level shifter 100 may be sufficient. However, in high-voltage systems, (e.g., Vhigh is 30V or more) the supply voltage may become more of a problem as present high performance transistors may not have a gate oxide able to standoff such high-voltages. High performance transistors may be defined as transistors with more current in less silicon area to have a low gate capacitance to operate at a higher speed.
Where the available supply voltages Vhigh exceeds maximum transistor Vgs, conventional level shifter solutions may operate with a standby current to maintain the level shifter output state. As this standby current is reduced to lower power consumption, other parameters such as shoot-through current, propagation delay, and output slew rate may be adversely affected. There exists a need for a level shifter for high-voltage applications, which has low shoot-through current and/or minimal standby current.